Boron is present in significant concentrations in seawater and in many natural brackish waters. Since high concentrations of boron are toxic to animals and many plants, boron is regulated in municipal water supplies. Boron concentrations for United States water supplies range from about 0.005 parts-per-million to about 2 parts-per-million, whereas boron concentrations for Los Angeles water supplies range from about 0.12 parts-per-million to about 0.281 parts-per-million. On the other hand, typical seawater has boron concentrations of approximately 4.6 parts-per-million.
Organizations such as the World Health Organization (WHO) and the Environmental Protection Agency (EPA) have proposed drinking water regulations having various limits for boron concentration. Particularly, the WHO has proposed a strict provisional boron concentration limit of 0.5 parts-per-million, while the EPA has set forth a more lenient boron limit of 3 parts-per-million for adults and 0.9 parts-per-million for children. Jurisdictions that have existing drinking water regulations for boron include Europe, Canada and Minnesota. Specifically, Europe has a boron limit of 1 part per million, Canada has a boron limit of 5 parts-per-million and Minnesota has a boron limit of 5 parts-per-million.
Treatment options for reducing boron concentration in drinking water include reverse osmosis and ion exchange processes. Several types of ion exchange resin are employed for boron removal, including: (1) the use of a boron specific resin consisting of a styrene-DVB backbone with N-methyl glucamine active sites; and (2) the use of a mixture of conventional strong-acid-strong-base mixed bed resins. One drawback of ion exchange processes is that the conventional resins remove all other ions (in addition to boron ions), thereby necessitating frequent regeneration with acid and caustic. Another drawback concerns the high cost of ion exchange processes.
Due to the above-identified drawbacks associated with ion exchange processes, reverse osmosis is the most frequently used treatment option for reducing boron concentration in sea and brackish waters. However, conventional seawater reverse osmosis systems do not meet the stringent provisional WHO boron limit of 0.5 parts-per-million. In fact, typical seawater reverse osmosis systems produce permeate with boron concentrations in the range of about 1.0 parts-per-million to about 1.2 parts-per-million. Moreover, boron concentrations levels tend to increase as the reverse osmosis membranes age.
Referring to FIGS. 1A (prior art) and 1B (prior art), most seawater reverse osmosis systems include a spiral wound membrane construction comprising a series of spiral wound elements 10 constructed with polyamide salt-rejecting membranes. Spiral wound element 10 comprises permeate tube 15, spacer 20, permeate carriers 25 and polyamide membranes 30. Pores within polyamide membranes 30 are constructed to reject ionic species such as sodium and chloride, while permitting water molecules to pass. Such seawater reverse osmosis systems are generally operated at pressures between 800 and 1000 psig, depending upon the salinity of the feed water and the precise manufacturing process. Water flux typically ranges from about 7 to 12 gallons per day per square foot of membrane surface area. Spiral wound elements 10 are expected to last at least three years before replacement.
Reverse osmosis systems are employed by many municipalities to produce drinking water from brackish water and seawater. In addition, some wastewater districts use reverse osmosis to reclaim wastewater for agricultural use or to prevent seawater intrusion into groundwater supplies. Reverse osmosis systems are also widely used by many industries to produce water with reduced levels of total dissolved solids. For example, reverse osmosis is used in the power industry for boiler makeup, in the electronics industry as a rinse for electrical components, in the chemical industry as a solvent and for rinsing, in the pharmaceutical industry for USP grade water production, in the mining industry for metals extraction, in the beverage industry for soft drink preparation, in the plating industry for plating bath preparation, and in the pulp and paper industry for paper manufacture.
Seawater reverse osmosis systems such as described in U.S. Pat. No. 6,709,590 may be constructed in multiple-pass arrays in order to meet the WHO's proposed boron limit. However, water costs for multi-pass seawater reverse osmosis systems are significantly higher than that of water costs for single-pass seawater reverse osmosis systems that do not meet the strict boron limit proposed by the WHO. In addition, some multiple-pass systems are augmented with boron-specific ion exchange resin for the purpose of converting non-ionized boric acid to negatively charged borate ion, which is more highly rejected.
U.S. Pat. No. 6,264,936 discloses the use of various organic biocidal chemicals including poly(hexamethylenebiguanide) hydrochloride (PHMB) for coating materials such as polysulfone and nylon sheets. However, this patent is not directed to the use of PHMB and other chemicals as a coating for membranes of a reverse osmosis membranes system to enhance boron rejection.
In view of the above, there exists a need for a single-pass reverse osmosis system capable of reducing the boron concentration in normal seawater to a level below 0.5 parts-per-million.
There also exists a need for a single-pass reverse osmosis system having membranes coated with chemicals such as PHMB to increase boron rejection capabilities.
Additionally, there exists a need for a single-pass reverse osmosis system having membranes coated with biocidal chemicals such as PHMB to reduce biofouling of reverse osmosis membrane surfaces.
There further exists a need for a single-pass reverse osmosis system that is less expensive than conventional ion exchange processes and multiple-pass reverse osmosis systems to reduce the cost of desalting seawater and brackish water.